overview map uses projection parameter or base projection before defaults

Addresses #2998:
- creates a view object using options.projection if specified, default EPSG:3857 otherwise
- if a map view exists upon adding the control to a map, the default is overriden with the map view projection

(shouldn't a system default projection be defined somewhere in ol.proj.Projection.defaults or the like?)
Here, a view object is created but is gets overwriten belew by a new instance if we have to use the parent projection.  Is there a better way to accomplish this, i.e. is there a way to use only one view instance ?